Microstructural evolution and mechanical behaviour of surface hardened low carbon hot rolled steel

Abstract

Surface hardening of low carbon hot rolled C–Mn steel has been successfully performed by high power diode laser with an achievable case depth of about 300μm. The laser treated samples have been characterised using optical microscope, scanning electron microscope, transmission electron microscope, and X-ray diffraction techniques. Higher hardness level is achieved in laser surface hardened zone (≈300HV) than in the base alloy region (≈140HV). The variation in hardness as a function of distance across the laser tracks is observed during multi-track laser hardening. Laser hardened steel sheets show enhanced mechanical strength (YS: 383–443MPa, UTS: 476–506MPa) with the lowering of percentage total elongation (23–28%) compared to the base alloy (YS: 351MPa, UTS: 450MPa and total elongation is 32%). Strain hardening exponent (‘n’) has been evaluated from true tensile stress–strain diagram and it shows a similar nature for both base alloy and laser treated steels. The microstructure in the base alloy region consists of a mixture of ferrite and pearlite, whereas predominantly lath martensite is present in the laser hardened surface layer. The improvement of mechanical strength is discussed in terms of the formation of this hardened layer on the surface.